This invention relates to sewing machines of the type employing pattern cam influenced mechanism for sewing stitch patterns in which not only the bight, i.e., the jogging of the needle laterally of the direction of work feed, is varied, but also the magnitude and direction of work feed may be cam influenced. More particularly, this invention pertains to a sewing machine of the above character which includes a four-step buttonhole mechanism, that is, operator influenced selection means for succesively setting the sewing machine controls to produce the four parts of a straight buttonhole, which include the bar tack at one end, zigzag stitches along one side in one direction of feed, the bar tack at the other end, and zigzag stitches along the other side in the opposite direction of feed.
A problem arises with sewing machines having both pattern cam control mechanism for the work feed and four-step buttonhole mechanism in providing for compatible adjustments of tolerance variations in each of these mechanism.
Because sewing machine work feed mechanisms may be more effective in feeding certain work fabrics in one direction than in the other, feed balancing adjustments are known for shifting the position of zero feed within the range of feed control, thus purposely to favor work feed in one direction so as to equalize the effectiveness of the work feeding mechanism in both directions of feed. For the buttonhole mechanism, however, known feed cam balancing means are not satisfactory for two reasons. For one reason, in buttonhole formation not only is equalization of the work feed in each direction of feed necessary, but it is also a requirement for quality buttonhole appearance that the work feed be maintained at zero during bar tack formation at each end of the buttonhole and, therefore, the shift of the position of zero feed incident to known feed balance control adjustment is detrimental to bar track formation when buttonholing. For the second reason, feed balance controls for required sensitivity are typically limited as to total possible adjustment which may be insufficient to reconcile the tolerance accumulation in a four-step buttonhole mechanism.